JEE Main 2013 — Classification of Elements and Periodicity in Properties Question with Solution

Electron affinity is defined as the change in energy (in kJ/mole) of a neutral atom (in the gaseous phase) when an electron is added to the atom to form a negative ion.

In other words, the neutral atom's likelihood of gaining an electron.
${\mathrm{Na}}_{\left(\mathrm{g}\right)}\to {{\mathrm{Na}}^{+}}_{\left(\mathrm{g}\right)}+{\mathrm{e}}^{-};$
$1^{st}$ ionisation energy ; ${\mathrm{\Delta H}}_{\mathrm{I}\mathrm{E}\left(1\right)}=+5.1\mathrm{eV}$
Electron gaining is the reversed reaction of ionization. Hence, electron gain enthalpy is ; 

$\left|{\mathrm{IE}}_{\mathrm{Na}}\right| = \left|{\mathrm{EA}}_{{\mathrm{Na}}^{+}}\right|$
${{\mathrm{Na}}^{+}}_{\left(\mathrm{g}\right)}+{\mathrm{e}}^{-}\to \mathrm{Na}$
So, ${\mathrm{\Delta H}}_{\mathrm{eg}}=-5.1\mathrm{eV}$